1. Field of Invention
A low-cost active tap for use in cable communication distribution systems and characterized by low power consumption, low signal distortion, and increased reliability.
2. Summary of the Background
In conventional coaxial communication distribution systems, signals are distributed along cable lines. Consumers located along the length of the cable run are supplied with signals via passive taps. FIG. 1 is a block diagram of a conventional four-port 24 dB passive tap device. The input signal flows in parallel to a power-passing choke PPC1 and to a series circuit comprising a first capacitor C1, a directional coupler DC, and a second capacitor C2. The directional coupler DC also outputs signals to a first splitter S1. This first splitter S1 feeds the signal then to a pair of second splitters S2A and S2B. The output of the second splitters S2A and S2B then flows to output ports O1, O2, O3, and O4. Signals may also flow upstream from output ports O1, O2, O3, and O4, through splitters S2A, S2B and S1 and directional coupler DC to a node upstream from the tap.
Because the components of the conventional tap are passive, the conventional tap attenuates the signal between its through-leg input and output as well as between its input and output to the subscribers. Thus, signal strength at the output of the device (e.g., 40.5 dBmV) is lower than at the input (e.g., 42.5 dBmV). The signal level at the output terminal of the directional coupler DC feeding S1 is predetermined by the design of the directional coupler. That is, a conventional DC16-type directional coupler provides a signal approximately 16 dB lower than the input signal level (i.e., 26 dBmV).
In order to maintain consumer cable TV and other communication signal quality, components of the conventional passive tap device are typically selected depending on the tap input signal level so that the signal level at each of output ports O1, O2, O3, and O4 is a minimum of 18 dBmV at the uppermost frequency. For example, as shown in FIG. 1, if the conventional passive tap device has an input of 42 dBmV, the directional coupler (DC-16) is selected to provide 26 dBmV at the input to splitter S1. This supplies 22 dBmV to S2A and S2B, which in turn supplies 18 dBmV to O1, O2, O3 and O4. This configuration also results in an output of the conventional tap device of 40.5 dBmV. As the signal level into the device decreases, the value of the DC also decreases and the through loss of the device increases.
FIG. 2 shows an idealized model of a conventional system for distributing signal from a source I1 to a number of consumers. Components of the conventional system include a source I1 in series with conventional passive taps P1-P18 and active line extenders LE1, LE2, and LE3. The passive taps P1-P18 each correspond to the device of FIG. 1, with the exception that the proper DC was selected to provide a minimum subscriber output of +18 dBmV. With this configuration, 18 passive taps with 4 output ports allow for 72 consumers to be supplied by source I1.
In FIG. 2 the figures of merit are composite triple beat (CTB); composite second order beat (CSO); and carrier-to-noise ratio (CNR). In FIG. 2, source I1 outputs a signal according the following profile: 54 dBmV at 862 MHz, 48 dBmV at 550 MHz, and 39 dBmV at 55 MHz. In addition, the signal output from source I1 has a CTB of −64.0 dB; a CSO of −60.0 dB; and a CNR of 51.0 dB. The signal propagates through the first five passive taps P1-P5 and coax during which the signals are attenuated to a level where no more subscribers can be supplied with +18 dBmV. So the signals must be amplified. The signals from the output of P5 are presented at the input to extender LE1. Line extender LE1 is configured to boost the signal to 49.5 dBmV at 862 MHz, 44 dBmV at 550 MHz, and 35 dBmV at 55 MHz. The signal passes through the next four passive taps to line extender LE2. Line extender LE2 is also configured to boost an input signal to 49.5 dBmV at 862 MHz, 44 dBmV at 550 MHz, and 35 dBmV at 55 MHz. The signal passes through the next four passive taps to line extender LE3. Line extender LE3 is also configured to boost an input signal to 49.5 dBmV at 862 MHz, 44 dBmV at 550 MHz, and 35 dBmV at 55 MHz. The signal then passes through the final five passive taps. For the reasons noted relative to FIG. 1, the system is designed so that the signal level at output ports O1, O2, O3, and O4 of each passive tap is on the order of 18 dBmV.
Throughout the propagation of the signal through the system, signal loss is introduced by the cable and passive taps. Line extenders are used to overcome the system losses but cause distortion to be added to the signals. Thus, at the output of line extender LE1, CTB equals −60.5 dB; CSO equals −59.0 dB; and CNR equals 50.4 dB. At the output of line extender LE2, CTB equals −58.0 dB; CSO equals −58.2 dB; and CNR equals 49.8 dB. At line extender LE3, CTB equals −56.0 dB; CSO equals 57.5 dB; and CNR equals 49.3 dB. Since all the devices after each LE are passive, the distortion numbers are the same for all taps after each LE, until the next LE. For example, the last 5 taps all have a CTB of −56.0 dB. The previous 4 taps have a CTB of −58.0 dB. CTB adds on a 20 log basis with the sum of the distortion of the source output plus the distortion generated by each LE. CSO and CNR add on a 10 log basis.
FIG. 3 illustrates signal characteristics for a more realistic cable television signal distribution system developed with conventional system modeling software. In FIG. 3, source N1 outputs a signal onto a first cable C1 of length 271 feet. The signal drops in strength due to line losses and is split by splitter S1 onto cables C2, C3, and C6. Cable C2 ends after 59 feet at a DC10 passive tap P1. Cable C3 extends 84 feet to a DC12 passive tap P2 which is connected to DC12 passive tap P3 via 230 feet of cable C42. Passive tap P3 is connected to a terminating passive tap P4. Cable C6 extends 538 feet to line extender LE1 which connects to a DC16 passive tap P5 via an additional 268 feet of cable C7 (the total length of cables C6 and C7 is 806 feet). Passive tap P5 connects to a DC8 directional coupler DC1 which is connected to a DC14 passive tap P6 (on the down-leg) and to cable C8, which extends 259 feet to a DC12 passive tap P7. Passive tap P7 connects to cable C9 which extends 277 feet to in-line equalizer EQ1, which is connected to a DC4 passive tap P8.
Input and output signal levels (in terms of dBmV) at 862 MHz/750 MHz/55 MHz, respectively, are shown in Table T1
TABLE T1DeviceInput Signal LevelOutput Signal LevelN1  --/--/--  50/48/36.5C1  50/48/36.546.5/44.7/35.6S146.5/44.7/35.641.8/40.2/32.138.1/36.7/28.6C238.1/36.7/28.637.3/35.6/28.4P137.3/35.6/28.4  20/19/11C341.8/40.2/32.140.7/39/31.8P240.7/39/31.8  20/18/11C4238.9/37.2/31.135.9/34.9/30.3P335.9/34.9/30.3  20/18/14C5234.1/32.8/29.725.5/24.8/27.5EQ225.5/24.8/27.525.1/23.9/17.5P425.1/23.9/17.5  17/15/12C638.1/36.7/28.628.6/27.8/26.4LE128.6/27.8/26.4  50/48/36.5C7  50/48/36.546.5/44.7/35.6P546.5/44.7/35.6  23/21/12DC145.4/43.7/35.3Through: 43.6/42.1/34.6Down-Leg: 37.4/35.7/27.3P637.4/35.7/27.3  20/19/12C843.6/42.1/34.640.3/39.2/33.7P740.3/39.2/33.7  20/19/14C938.5/37.1/33.134.9/34.4/32EQ134.9/34.4/3234.5/33.0/22.4P834.5/33.0/22.4  17/16/11
FIG. 4 illustrates the power consumption of the idealized system of FIG. 2. As shown in FIG. 4, power consumption of active line extenders LE1, LE2 and LE3 is 28.5 watts each, resulting in a total power consumption of 85.5 watts. Based on a power supply located at the Source I1, and a 0.500″ PIII DC loop resistance of 1.2 Ohms/1000 ft, the power dissipated by the cable is 1.95 Watts. Power consumption of the conventional taps is minimal because the only component that would dissipate any power in the tap is the RF Choke (PPC1 in FIG. 1). The resistance of the RF Choke, which allows the power to pass through the tap, is typically less than 0.01 Ohms. Thus total power consumed in the idealized conventional system shown in FIG. 4A is 87.15 watts (85.5+1.95 watts). Cable TV (CATV) operators are extremely interested in reducing power consumption in their distribution systems. Even a savings of 3 watts, which can save $15 in power costs over 5 years, is considered very favorably.
In 1991, Chiddix and Vaughn proposed a concept of an active tap. This proposal was motivated by the difficulties at the time of increasing cable services by just increasing the bandwidth of the distribution plant. The intent of the paper was to encourage manufacturers to help solve these problems by building an active tap that would extend services to a larger number of customers without undue signal distortion. (In 1991, no CATV distribution equipment was being built with GaAs active devices or bipolar transistors with an fT of 6 GHz. Gain stages at the time that were rated above 550 MHz provided additional bandwidth, but had poor distortion performance.) A small segment of a cable system owned by Chiddix's employer was built with a bandwidth of 1 GHz to test pay-per-view markets. The system was built in the traditional way (amplifiers and passive taps). The experiment was a marketing success, but the amplifier distortion performance was insufficient, thus making the system impractical.
For many reasons, the active tap proposed by Chiddix and Vaughn were not reduced to practice. First, to achieve suitable performance the equipment power requirements would have increased and the power passing circuitry did not exist. At the time, it was not possible to make power-passing circuitry capable of passing in excess of 15 amps without saturating the ferrite coil form, causing modulation of the RF signals. Second, the power consumption of the active tap concept of Chiddix and Vaughn, if ever reduced to practice, would have resulted in a system that was not cost-effective to operate. For example, if, as described, a gain stage were used at each subscriber port, a 4-port tap would consume in excess of 32 watts (whereas the device disclosed in the following detailed description will provide the same functionality while requiring only 0.5 watts, providing a $150 savings in powering over five years for every active tap).
Third, system managers are graded on subscriber minute outages. A 1991 cable system could have 100,000 or more subscribers. The signals for these subscribers would all originate from the head-end and begin by flowing through a single amp. If that amp fails, the network suffers 100,000 subscriber minute outages for every minute the system is down. The Chiddix-Vaughn proposal did not reduce the problems associated with amplifier cascades, but added to the system downtime probability by increasing the number of system power supplies required. Power outages at supply locations are a major source of system outages.
Fourth, no cable systems have ever been built with 550 MHz, or 1 GHz bandwidth, as suggested by Chiddix and Vaughn. No device was available then or is available even today, that could operate at 55 dBmV output with 151 analog channels. Furthermore, Chiddix and Vaughn's proposal to power the active tap from one of possible four subscriber's houses adds cost, complexity and unreliability to the system. Also, if the subscriber providing power disconnects, a major reconfiguration of the system will be needed.
Fifth, a concern of an active or passive tap system is reverse signal levels. Devices installed in houses have a maximum output of +55 dBmV. The described reverse injection of 30 to 40 dB in the system of Chiddix and Vaughn would not provide enough signal to drive the reverse laser circuitry at the fiber node. In real applications the directional coupler is selected to satisfy the reverse signal requirements.
Sixth, the system of Chiddix and Vaughn is lacks control of signal level change due to system ambient temperature changes. The lack of amplitude control and cable versus frequency equalization cause the reach of the system to be reduced and could cause unacceptable signal to distortion levels at the subscriber ports.
Finally, to be cost-effective the system proposed by Chiddix and Vaughn required the unit to have a selling price of $100. The cost to build the unit was estimated to be in excess of $120. Manufacturing cost of an off-premise converter which contained some of the components required, such as microprocessors, hybrids, PIN diode switches, DC power supply, was $235 per subscriber. Because manufacturing costs were estimated to be more than double what was required to be cost-effective, no devices were ever built.
Because the technology of the day did exist to build the device suggested by Chiddix and Vaughn, because the overall system was economically and operationally inoperative, and because since 1991 the utilization of fiber optics has increased and digital compression of TV channels have provided a great deal of additional service capacity to the systems, and improvement in traditional gain stages, including GaAs active devices has provided a means of building systems with a bandwidth of 870 MHz, the proposal of Chiddix and Vaughn was not reduced to practice.
Thus, what is desired, as discovered by the present inventors, is a capability for distributing coaxial communication signals at greatly reduced power consumption, and system reliability, where signal levels and quality are equal to or better than is possible with conventional amplifier and passive tap systems and where the amplification is provided at the tap and characterized by low power (less than 1 Watt), low noise figure (less than 3 dB), high bandwidth (typically 20-1.5 GHz), and high gain (e.g., input to subscriber output port gain of as much as 22 dB).